Several marine equipment suppliers are now manufacturing "fly by wire" steering and/or engine control systems for marine vessels. Such systems have merit because they simplify the installation and reduce the costs associated with auxiliary control stations. For example, flying bridge and portable remote control stations are simpler to install with wiring than with the plumbing and cabling associated with hydraulic- and cable-actuated control systems.
However, fly by wire systems are not without their problems. Transferring control among multiple helms and an autopilot requires some sort of synchronization of the multiple possible steering commands to each other and to the actual rudder position. (The descriptions presented in this application refer, for purposes of convenience, to a rudder of a marine vessel, although they are also applicable to any marine vessel controllable turning moment generator such as an outboard or outdrive steering angle actuator.) Without synchronization, when control is transferred from one steering input. device to another, the rudder actuator attempts to "jump" to the newly commanded position, creating what is referred to as a control "bump." These problems result from a traditional steering systems paradigm, in which an absolute wheel angle causes a corresponding rudder angle, e.g., a centered (between helm stops) helm rotation angle causes a zero rudder deflection, and a large helm rotation angle (at the helm stop) causes a fully deflected rudder in a corresponding direction.
FIG. 1 represents operational control states found in typical prior art autopilot systems in which a helmsman must steer to a desired heading and press a button to place the autopilot (AP) in an engaged state 10. To place the autopilot in a disengaged or standby state 12, the helmsman must press a standby button, or in some cases, disengage a clutch or turn the helm. Some prior autopilots will revert to engaged state 10 if the helmsman steers back to the original heading. Many prior autopilots further include a power steering feature in which the rudder angle or heading setpoint can be controlled by a handheld remote control or by a knob on the autopilot control panel.
FIG. 2 represents a typical prior art hydraulic steering system in which a helm 22 rotates a helm pump 24, and an autopilot pump motor 26 rotates an autopilot pump 28. Either autopilot pump motor 28 or helm pump 22 can supply fluid to a steering cylinder 30 that actuates a rudder 32. No bump occurs in such a steering system if the autopilot system is engaged when autopilot pump motor 26 is stopped (i.e., starting rudder command equals the current angle of rudder 32). Likewise, no bump occurs when the autopilot is disengaged because rudder 32 simply responds to rotations of helm pump 24. Moreover, if the autopilot is engaged while helm 22 is rotating, the normal response is for the autopilot to correct by causing autopilot pump 28 to subtract fluid from steering cylinder 30 to compensate for fluid added by rotation of helm pump 24. Because of the hydraulically coupled synchronization of such steering systems, there are many known techniques by which helm 22 can automatically override the autopilot. It should be noted that when steering cylinder 30 reaches its stops, helm 22 is also stopped. Of course, steering system 20 may have multiple helms and autopilots hydraulically coupled to steering cylinder 30. With suitable electronic inputs to the autopilot, autopilot pump 28 is usable as a power steering device.
There are previously known non-hydraulic techniques for synchronizing helms and autopilot systems. Referring to FIGS. 3 and 4, U.S. Pat. No. 5,107,424 for CONFIGURABLE MARINE STEERING SYSTEM ("Bird et al.") describes an example of a prior fly by wire steering system 40 having multiple steering devices 42 that are selectable by an input selector 44. To prevent steering angle bumps in steering system 40 when input selector 44 selects a different one of steering devices 42, the newly selected device is first electronically initialized to the current rudder angle. Moreover, mechanical stops associated with steering devices 42 were eliminated so that any newly selected steering device can simply add to or subtract from the rudder position commanded by the previously selected steering device. Accordingly, synchronization among steering devices 42 in steering system 40 employs continuously rotatable, incremental steering devices in combination with steering device initialization.
Bird et al. recognized that incremental steering commands can accumulate to an indefinitely large number. Therefore, each input device limits its output to the maximum deflection of the rudder. FIG. 4 shows that a limiter 50 in controller 46 prevents a rudder actuator 48 from being commanded beyond its mechanical stops. A rudder angle transducer 52 closes the steering servo loop.
Bird et al. implemented helms 54 and 56 with incremental optical encoders driving associated pulse-counting up/down accumulators. However, whenever one of helms 54 or 56 is selected, its up/down accumulator must be reset to zero, making each of helms 54 and 56 yet another initialized device.
What is needed, therefore, is a marine vessel fly by wire steering system that automatically and seamlessly transfers steering control among multiple steering devices, which may include one or more autopilots or helms, without necessarily requiring manual steering device selection.